Field of the Invention
The present invention relates to an image heating apparatus to be included in an image forming apparatus employing an electrophotographic system.
Description of the Related Art
Hitherto, as an image heating apparatus to be included in an image forming apparatus adopting an image forming process employing an electrophotographic system, an electrostatic recording system, or other systems, such as a copying machine and an LBP, there has been used one employing a film fixing system. In the image heating apparatus employing the film fixing system, a fixing film and a pressure roller (pressure member) are arranged in pressure-contact with each other, and a heating element for heating the fixing film is arranged to be brought into close contact with an inner surface of the fixing film at a portion at which the fixing film and the pressure member are opposed to each other (nip portion). As the heating element, a ceramic heater is generally used, which is obtained by forming a heat generating resistor on a substrate made of a ceramics such as alumina or aluminum nitride. The heat generating resistor, formed on the ceramic heater, inputs a current in a state obtained by subjecting a primary current flowing from an electrical outlet to power control by a control method such as wavenumber control or phase control in a power supply circuit, thereby generating heat and heating an image. The heating element is supported by a holder (support member) made of a resin or the like, and a temperature detecting element, a safety element, and the like are arranged in contact thereto. Those elements have functions such as input power control based on the detected temperature and current interruption during an abnormal temperature rise.
When the above-mentioned film-heating fixing device including the heating element carries out an operation of heating and fixing an unfixed toner image onto a recording material having a small width in a longitudinal direction (a direction orthogonal to a conveyance direction of the recording material, width direction of the recording material) (small-sized sheet), there is a difference in the heat radiation amount of the film in the longitudinal direction. In other words, a part of the film that is brought into contact with the recording material loses heat to the recording material, but a part that is not brought into contact with the recording material does not lose heat to the recording material. Therefore, at the nip portion, there is caused a phenomenon that the temperature of a region through which the recording material does not pass (non-sheet-passage portion) is higher than the temperature of a region through which the recording material passes (sheet-passage portion), which is what is called non-sheet-passage portion temperature rise. The occurrence of the non-sheet-passage portion temperature rise causes image defects due to temperature unevenness at the nip, wrinkled sheets due to thermal expansion of the pressure roller in the non-sheet-passage portion, sheet conveyance failure, and the like. Further, thermal deterioration of parts of the film and the pressure member corresponding to the non-sheet-passage portion may progress, which may result in damage.
In order to solve the problem of the non-sheet-passage portion temperature rise, in Japanese Patent Application Laid-Open No. H11-84919, there is proposed a configuration in which a high heat conductive member is mounted between the heater substrate and the support member, to thereby obtain a uniform heater temperature distribution in the longitudinal direction. Further, in Japanese Patent Application Laid-Open No. 2014-123100, in order to secure safe use of the high heat conductive member, there is proposed a configuration in which two high heat conductive members are arranged in the longitudinal direction, and one high heat conductive member is brought into contact with a thermistor, while the other high heat conductive member is brought into contact with a fuse. In this configuration, the primary side and the secondary side of the power supply circuit of the image forming apparatus are electrically separated from each other.
In the configuration in which a single high heat conductive member continuous in the longitudinal direction is used as in Japanese Patent Application Laid-Open No. H11-84919, a metal plate (for example, aluminum plate) used as the high heat conductive member is formed into an elongated shape in accordance with the size of the heater, and hence there is a fear of the occurrence of warpage. When warpage occurs, the degree of contact of the high heat conductive member to the heater substrate may become non-uniform in the longitudinal direction.
In the configuration in which a plurality of high heat conductive members are arranged as in Japanese Patent Application Laid-Open No. 2014-123100, the fear of a reduction in the degree of contact between the high heat conductive member and the heater substrate due to the occurrence of warpage is reduced, but a gap is formed between the two high heat conductive members, and hence there arises a new fear in heat conduction uniformity.